Waiting for GUI events and IOCP notifications at once

Question

Can I wait for GUI events — that is, pump a message loop — and on an I/O completion port at the same time? I would like to integrate libuv with the Windows GUI.

Answer 1

There are two solutions that I know of. One works on all versions of Windows, but involves multiple threads. The other is faster, but only supports Windows 10+ (thank you @RbMm for this fact).

1. Another thread calls GetQueuedCompletionStatusEx in a loop, and sends messages to the main thread with SendMessage. The main thread reads the messages from it's message loop, notes the custom message type, and dispatches the I/O completions.

   This solution works on all versions of Windows, but is slower. However, if one is willing to trade latency for throughput, one can increase the GetQueuedCompletionStatusEx receive buffer to recover nearly the same throughput as the second solution. For best performance, both threads should use the same CPU, to avoid playing cache ping-pong with the I/O completions.

2. The main thread uses MsgWaitForMultipleObjectsEx to wait for the completion port to be signaled or user input to arrive. Once it is signaled, the main thread calls GetQueuedCompletionStatusEx with a zero timeout.

   This assumes that an IOCP that is used by only one thread becomes signaled precisely when an I/O completion arrives. This is only true on Windows 10 and up. Otherwise, you will busyloop, since the IOCP will always be signaled. On systems that support this method, it should be faster, since it reduces scheduling overhead.